Electrostatic printing



This invention relates to electrostatic printing and particularly, but not necessarily exclusively, to improved photosensitive compositions and recording elements for electrostatic printing and to improved methods of electrostatic printing utilizing said improved recording elements. j An electrostatic printing process is that type of process for producing a visible record, reproduction, or copy which includes, as an intermediate step, converting a light image or electric signal into an electrostatic charge pattern on an electrically-insulating base. The process includes the conversion of the electrostatic charge pattern into a visible image by the application thereto of electrostatically-attractable particles. An electrostatic printing process using a photoconducting insulating stratum to convert a light image into an electrostatic charge pattern is described by C. J. Young and H. G. Greig in Electrofax Direct Electrophotographic Printing on Paper, RCA Review, volume XV, No. 4, pages 469 to 484 (December 1954), hereinafter referred to as the Young and Greig publication.

A typical electrostatic printing process utilizing a photoconducting insulating stratum includes first producing a blanket electrostatic charge on the surface of the photoconducting stratum. The electrostatic charge may be stored on the surface for a time in the dark. The rate at which this stored electric charge is dissipated when the stratum is maintained in darkness is hereinafter referred to as the dark decay of the stratum. Within the period in which a substantial charge remains, a light image is focused on the charged surface, discharging the portions of the surface irradiated with light, leaving the remainder of the surface in a charged condition, and thereby forming an electrostatic image thereon The electrostatic image is rendered visible by applying to the electrostatic image a developer substance, such as a pigmented thermoplastic resin powder, which is held ole..- trostatically to the charged areas of the surface. The visible image thus formed may be fixed directly to the surface, for example, by fusing the image thereto.

One type of recording element usable in the foregoing electrostatic printing process comprises a cellulosic sheet coated with a particulate photoconductor dispersed in an electrically-insulating, film-forming vehicle, for example, ordinary paper coated with particulate photoconducting white zinc oxide suspended in a silicone resin. Such recording elements are relatively cheap, flexible, and are prepared in standard web coating machines from solvent coating mixtures which are air dried.

Silicone resins, as well as many other resins taught in the prior art, are thermosetting. As previously prepared for use in electrostatic printing, the silicone resin is not set or is uncured and, as a result, retains its flow and adhesive properties. One difliculty of such coated paper is that it blocks when wound tightly in rolls. Blocking is the adherence of adjacent layers of paper due to the adhesive properties of the coatings.

Another difliculty occurs in high speed printing machines where the recording element is a continuous web drawn through the machine by a pair of drive rollers located subsequent to the step of fixing. The small amount of heat remaining in the recording element after 2,990,279 Patented June 27, 1961 fixing, further softens the coating which frequently adheres to the drive rollers or is displaced by the drive rollers.

One solution to the difliculties' is to set or cure the silicone resin. This solution is impractical because (1) the heat required for curing deteriorates the other components of the recording element, (2) the cost of producing such recording element is prohibitive, and (3) a cured silicone resin is usually brittle and cracks when flexed.

An object of this invention is to provide improved photosensitive compositions and improved recording elements for electrostatic printing.

Another object is to provide improved recording elements for electrostatic printing which are non-blocking, cheap, flexible, and easily prepared in a standard web coating machine.

A further object is to provide improved recording elements particularly adapted for use in high speed electrostatic printing machines.

The improved compositions for electrostatic printing comprise, generally, a particulate photoconductor suspended in an electrically-insul'ating, film-forming vehicle, wherein said vehicle includes at least two compatible, hydrophobic, solvent-soluble resins in proportions to provide the desired non-blocking and non-adhesive characteristics to said composition, one of said resins being nonadhesive at the temperature at which the recording element is used and the other of said resins having a low electrical conductivity. By using a combination of resins as the vehicle, it is possible to adjust the physical and electrical properties of the material over a wide range, and particularly to provide a non-blocking material while retaining other desirable physical and electrical properties as well as low cost and ease of manufacture.

An example of a recording element of the invention comprises a stratum comprising a major proportion of a particulate photoconductor, such as photoconducting zinc oxide, suspended in an electrically-insulating, film-forming vehicle, wherein said vehicle is a mixture of a high styrene-butadiene rubber resin, such as Pliolite, anda hard hydrocarbon resin such as Piccopale. The invention includes recording elements comprising the foregoing stratum which may be self-supporting or a stratum which is supported upon a backing, such as paper. A

The foregoing objects and other advantages are more fully described in the following detailed description when read in conjunction with the accompanying drawing in which: I

FIGURE 1 is a sectional, elevational view recording element according to the inventio FIGURE 2 is a partially-schematic, sectional view of an apparatus for producing a blanket electrostatic charge upon a recording element produced by the method of the invention,

FIGURE 3 is a partiallysectional, elevational view-of an apparatus for projecting a light to forma contact image upon the charged recording element of FIGURE 2, and

FIGURE 4 is a sectional view of an apparatus for developing an electrostatic image upon the recording element produced in FIGURE 3. i

Similar reference characters are applied to similar structures throughout the drawings.

A detailed example for preparing and using an electrophotographic recording element in accordance with the present invention will now be given.

of a typical EXAMPLE 1 Referring to FIGURE 1, a recording element comprising a backing Z1 and a photoconducting coating 23 thereon according to the invention is prepared as follows. A coating mixture is prepared of the followingformulation inparts by weight:

Rubber Co., Akron, Ohio 9 F21 Plasticizer, marketed by the National Lead 00.,

New York, N.Y. 1.5 Toluene 100 The mixture is ball-milled to a smooth uniform consistency and is coated on the surface of a paper backing 21, such as Gleamcoat CIS-40 marketed by Crocker, Burbank Papers, Inc., Fitchburg, Massachusetts to a thickness to provide a final dry coating of about 0.001 inch. Any conventional coating process may be used. One convenient coating process is by flow-coating. Other coating processes may be by spraying, dip-coating or spin-coating. The coating 23 is air dried. Heat may be used to accelerate the drying step. Upon drying, the recording element is ready for use in electrostatic printing.

The recording element described is non-blocking, cheap, flexible and easily prepared in standard web coating machines. It is particularly adapted for use in high speed machines by providing a fast response in the electrostatic printing process. In addition the recording element is white in color and does not contain any toxic ingredients.

The recording elements of the invention may be used for electrostatic printing by any of the methods described in the Young and Greig publication op. cit. By way of example and referring to FIGURE 2, the recording element of Example 1 may be utilized in an electrostatic printing process according to the following steps. The recording element is placed with the backing 21 upon a grounded metal plate 25 and in darkness, an electrostatic charging device 61 passed over the photoconducting coating 23 to provide an electrostatic charge thereon. The charging device 51 may comprise an array of fine wires 53 mounted near the grounded metal plate 25. A source of DC. voltage is connected between the wires 53 and the grounded plate 25 to provide a negative charge on the wires with respect to grounded plate 25. The voltage should be sufliciently high to cause a corona discharge adjacent the wires. It is preferred to apply about 6000 volts with respect to ground to the wires 53 when charging the surface of the recording element of Example 1. The surface of the recording element passing under the charging device 51 becomes charged negatively when the preferred voltage applied. The apparatus and process may produce a blanket positive charge if the polarity of the wires 53 is positive with respect to the grounded plate 25.

The next step in the process is to discharge selected areas of the charged surface of the recording element in order to produce an electrostatic image thereon. Referring to FIGURE 3, this may be accomplished by placing a photographic transparency 61 upon the charged surface of the coating 23 and exposing to light derived, for example, from a lamp S9 in the manner of conventional contact printing. An exposure of about 1 second from a 100 watt tungsten lamp about 24 inches from the recording element for a normal density photographic transparency has been found to be adequate. The light image may be produced by any of the ordinary photographic processes as by projection, contact, or reflex. Any type of electromagnetic radiation may be used provided a portion thereof falls within the spectral sensitivity of the photoconducting coating 23. In the example, visible blue and ultraviolet light may be used. For other photoconducting strata, visible light, infra-red, ultraviolet or X-rays may be used. Wherever the light strikes the surface of the photoconducting coating 23, the electrostatic charge thereon is reduced or removed. This leaves an electrostatic image or pattern of charges corresponding to the non-illuminated areas of the light image.

The electrostatic image may be stored for a time if desired. Ordinarily the next step is to develop the electrostatic image with a finely-divided developer substance such as a finely-divided powder or an ink mist. Referring to FIGURE 4, development of the electrostatic image is preferably accomplished by maintaining the recording element in darkness and passing a developer brush 55 containing a developer powder across the surface of photoconducting coating powder 23 bearing the electrostatic image. Areas 27 of developer powder are deposited on those areas of the surface retaining an electrostatic charge. The developer brush comprises a mixture of magnetic carrier particles, for example powdered iron, and the developer powder. The mixture is secured in a magnetic field by a magnet 57 to form a developer brush.

A preferred carrier material for the developer mix consists of alcoholized iron, that is, iron particles free from grease and other alcohol-soluble impurities. These iron particles are preferably relatively small in size, being in their largest dimension about .002 to .008". Satisfactory results are also obtained using a carrier consisting of iron particles of a somewhat wider range of sizes from about .001" to .020".

A preferred developer powder may be prepared as follows: A mixture comprising 200 grams of 200 mesh Piccolastic resin 4358 (an elastic thermoplastic resin composed of polymers of styrene, substituted styrene and its homologs) marketed by the Pennsylvania Industrial Company, Clairton, Pa., 12 grams of Carbon Black G, marketed by the Eimer and Amend C0,, New York, N.Y., 12 grams of spirit Nigrosine S.S.B. marketed by the Allied Chemical and Dye (30., New York, N.Y., and 8 grams of Iosol Black, marketed by the Allied Chemical and Dye Co., New York, N.Y. are thoroughly mixed in a stainless steel beaker at about 200 C. The mixing and heating should be done in as short a time as possible. The melt is poured into a brass tray and allowed to cool and harden. The hardened mix is then broken up and ball-milled for about 20 hours. The powder is screened through a 200 mesh screen and is then ready for use as a developer powder. This powder takes on a positive electrostatic charge when mixed with glass beads or iron powder. It therefore develops an electrostatic image composed of negative charges. Two to four grams of the developer powder and grams of the magnetic carrier material are blended together giving the preferred developer mix. Other ratios may be used.

The developer powder may be chosen from a large class of materials. The developer powder is preferably electrically-charged to aid in the development of the electrostatic latent image. The powder may be electricallycharged because the powder (1) is electroscopic, or (2) has interacted with other particles with which it is triboelectrically active or (3) has been charged from an electric source such as a corona discharge. Examples of suitable developer powders are powdered zinc, powdered copper, carbon, sulfur, natural and synthetic resins or mixtures thereof.

The developer powder may be applied to the electrostatic image in other ways, for example, it may be dusted on to the image, or it may be mixed with glass beads, magnetite, or other suitable carrier particles, and the mixture then brought into contact with the surface of the photoconducting stratum. The beads serve merely as a temporary carrier, releasing the powder particles upon contact with the charged surface.

The type of developer powder described is a positivelycharged powder and will adhere readily to negativelycharged areas of the electrostatic image. In the developed image described, the developed areas of the image correspond to the non-illuminated portions of the light image. If the photoconducting stratum is charged positively, and the same steps are carried through as above-described, a reverse image is obtained. If a negatively-charged powder is used in place of the positively-charged powder,

then a reverse image is obtained in the first case and a positive image is obtained in the alternative case.

The developed image 27 is now fixed to the photoconducting coating 23. If the developer powder or the vehicle in the photoconducting coating 23 has a relatively low melting point, the developed image may be fixed by heating, for example with an infra red lamp, to fuse the powder to the surface. Sulfur or synthetic thermoplastic resin powders may be fixed in this way. Alternatively, the developed image 27 may be pressed into the coating 23. Another method of fixing the developed image 27 is to apply a thin coating of a solvent for the material of the developed image 31. The solvent softens the developer powder particles and causes them to adhere to one another and to the photoconducting coating 23. Alternatively, a solvent may be used to soften the photoconducting stratum 23 and cause the developed image to adhere thereto. Upon standing, and preferably with the application of a slight amount of heat, the solvent is evaporated from therecording element.

While the backing 21 of Example 1 is a particular brand of paper, any substrate may be used. Thus, one may use any cellulosic material such as paper, cellophane, cellulose acetate or mylar metallic material such as copper, aluminum or brass, or mineral material such as glass or mica. The substrate or backing 21, may be in any desired shape or configuration. It is preferred, although not necessary, that the substrate have a higher electrical conductivity than the final photoconducting coating 23.

The photoconducting coating 23 must be a material which exhibits a substantial change in electrical conductivity upon exposure to light, such that an electrostatic charge stored on the surface thereof may be discharged thereby. The photoconducting coating 23 determines the spectral response, the speed of response and the contrast characteristic of the recording element. By a proper choice of materials for the photoconducting coating 23, any spectral response, speed of response or contrast characteristic over a wide range may be obtained. Many different powdered photoconductors may be used in the photoconducting stratum in place of the white photoconducting zinc oxide. For example, one may use a dye-sensitized white photoconducting zinc oxide as described in the cited Young and Greig publication, cadmium-zinc sulfide phosphors, or panchromatically-sensitive or bufi zinc oxide, as described in US. Patents 2,727,- 807 and 2,727,808 to S. M. Thomsen. The photoconducting stratum may be any of the photoconducting coatings described in the Young and Greig publication, ibid.

In the research work to develop an electrophotographic recording element described in the Young and Greig publication, op. cit., it was noted that several of the zinc oxides tried for making recording elements, did not print or printed very poorly. Most of these were American Process zinc oxides produced directly from the zinc ore. Zinc oxides which produce recording elements that print well or poorly are almost identical in appearance.

Several tests have been devised to distinguish the photoconducting zinc oxides which are useful in the photoconducting coating 23 from the zinc oxides which print poorly as follows:

Test 1.-A mixture was prepared comprising about milligrams of dry zinc oxide powder and a few drops of an 80% solution of silicone resin in xylene (G.E. SR-82, marketed by the General Electric Company, Silicone Products Division, Waterford, N.Y.), diluted with toluene in the ratio 60 grams solution to 105 grams toluene. The mixture was coated on filter paper and dried to produce a dry coating over an area about 0.25 inch in diameter. The dry coating was cooled to about l90 C. and examined in light from a mercury vapor lamp having a maximum output at about 3650 A. The zinc oxides which produce printable coatings produce a lavender or orange luminescence by this test. Other zinc oxides exhibit a green or yellow luminescence.

Test 2.About 0.25 gram of dry zinc oxide powder was placed in a silica boat. The boat was inserted into a silica tube and the system flushed with hydrogen gas. The tube and boat were fired for about 5 minutes at about 1000 C. in a stagnant hydrogen atmosphere. The boat was cooled in hydrogen to room temperature. The fired zinc oxide was examined in light from a mercury vapor lamp having a maximum output at about 3650 A. The zinc oxides which produce printable coatings luminesce brightly. Other zinc oxides luminesce weakly or not at all.

The table compiles the results of the two tests on typical commercially-available zinc oxides and indicates the correlation between these tests and the printability of the zinc oxides used in the coatings of the invention. The zinc oxides marked G in column 3 print well and are preferred in the photoconducting coatings 23.

Table Zinc Oxide Test 1 Test 2 Test 3 Mallinckrodt: 1

1. Analytical Reagent lavender bright greenm" G WXAV-l. 2. Analytical Reagent 0r.-lavender .d0 G

WYEB. 3. Ultrapure Y-lOOO. orange grey green. G 4. light purple bright green G 5. lavender G 6 0 G WYBA. Eagle-Picher: 2

7'. USP G 8. #109-.- G 9. #310 G 10. #417 P 11. #730.-. P 12. #7-169 WS White Seal. grey lavenden bright green G 13. #720 S-7083 grey green very dull green P AZO: 3

14. #11 green do P 15. #22. d0 nearly dead I 16. #33- 17. #44 18. #66 NJ. Zinc 00.: 4

19. Kadox 15 20. Spec. Pure Z4763"..- 21. XX 504 22. Special #3 (American Prod). 23. Spec. Pure #500 #(99920). 24. Florence Green Seal Florence Red Seal 26. Florence White Seal... d0 27. O.E. 8099-5-3 light green 28. O.E. 8099-3 dull brown green. Fischer: 5

29. Cat. #Z-51 tech. dry. yellow very dull green P proc. 30. Cat. #Z-49 USP dry steel gray bright green... G

proe. 31. Cat. #Z-53 lavender "do G Miscellaneous:

32. SpexStandard (spec. d0 .do G

pur 33. .Tohns0n-Matthey G (spec. pure) #9321 Cat. #JM155. 34. B and A (wet propeach do G (11285) 0.1 #2449 Lot 35. on. Smith 99.85% light purple do G 6 Spex Industries, Queens Village 27, N.Y. 7 Johnson, Matthey and 00., 73 Hatton Garden, London, England. 8 Allied Chemical and Dye Corp. General Chemical Division, New York, N .Y.

9 G. Frederick Smith Chemical 00., Columbus, Ohio.

The vehicle for the photoconductor in the photoconducting coating should have the following characteristics:

(1) Hydrophobic.

(2) Electrically-insulating.

(3) Film-forming and good wetting of the photoconductor.

(4) Non-acidic and non-reactive with the photoconductor.

(5) Ability to hold high proportions of photoconductor.

(6) Light in color, preferably colorless.

(7) Non-toxic.

(8) Low in cost.

(9) Non-blocking after coating.

It has been found that very few vehicles possess all of the foregoing characteristics. However, by mixing two or more vehicles, one may obtain a balance of desired characteristics. Mixtures of vehicles must, of course, be compatible with each other in the presence of the photoconductor. In addition, the properties of mixtures of vehicles are not predictable from the properties of the vehicle components. But it has been found unexpectedly that mixtures of the above-described components which are non-blocking are operative in electrostatic printing.

Example 1 is representative of the system of mixtures of a rubber resin (Pliolite) and a hard hydrocarbon resin (P'iccopale). Both of these classes of resins are well known to persons skilled in the art of chemistry. Pliolite SSD is a styrene-butadiene copolymer rubber resin. Increasing the proportion of Pliolite in the coating, increases the flexibility and toughness and reduces tackiness of the coating. Piccopale 100 is a hard hydrocarbon produced by the polymerization of unsaturates derived from the deep cracking of petroleum. Increasing the proportion of Piccopale in the coating, increases ease of dispersion of zinc oxide, reduces the tendency to form orange peel texture, and improves the drying characteristics of the coating. Piccopale refers to the polymerization product obtained by catalytic cracking under carefully controlled conditions, a mixture of monomers having an average molecular weight of approximately 90, and composed essentially of dienes and reactive olefines to produce a hard solid resin with a resulting average molecular weight approximately 1100 and generally approximating 100 C. ball and ring softening point. Piccopale appears to be methylated paraffin chains containing only a limited amount of unsaturation either of trans type II or terminal double bonds. The large number of tertiary hydrogens produce misleading results when the material is analyzed for unsaturation by the Wijs iodine method unless a correction is made for substitution. An iodine value of 120 by the usual method when corrected for the presence of hydrogen halide from substitution gives a corrected iodine value of 23 which agrees in principle with a koppeschaar bromination number of approximately 7. The ebullioscopic molecular weight of 1050, taken with the corrected iodine number, would indicate 1.9 double bonds per mol, or from the bromine number .9 double bond per tool.

The combination of a rubber resin and a hard hydrocarbon resin, and particularly Pliolite and Piccopale, may be used in any proportion with respect to one another to impart more or less of the desired characteristics. All of the mixtures with Pliolite and Piccopale are operative in the above-described electrostatic printing process.

A plasticizer may be used in conjunction with the vehicle to enhance foldability, flexibility and pliability to the final coating. The choice of the plasticizer is determined by the vehicle used. Some of the common types of plasticizers that may be used include phosphates, such as tricresyl phosphate and triphenyl phosphate; phthalates, such as di-Z-ethylhexyl phthalate; and petroleum derivatives such as Sovaloid C manufactured by Socony- Vacuum Oil Company, New York, N.Y. The quantity of plasticizer used is determined by the degree of flexibility required in the coating and by the particular vehicle used. The quantity of plasticizer may be present in an amount between zero and eightly percent by weight of 8 the vehicle. However, a quantity of plasticizer is preferred.

The proportion of powdered photoconductor to vehido in the final coating may vary over a very wide range. The preferred ranges are to 900 parts by weight of photoconductor to 100 parts by weight of vehicle. The optimum proportion will depend upon the nature of the photoconductor, the nature of the vehicle and the results desired.

The speed of response of the recording element de pends upon the nature of the photoconducting material, the nature of the vehicle, and the ratio by weight of photoconductor to vehicle. Since the speed of response depends upon a number of characteristics, almost any desired response may be obtained by the proper selection of materials and composition. A proper selection of materials and compositions will also determine how long an electrostatic image may be stored on the surface of the photoconductive coating since storage of the electrostatic image depends upon the electrical resistivity of the material. Generally, the higher the resistivity of the coating the longer the storage time for the material.

A dye for sensitizing the photoconductin-g coating may be incorporated into the coating when the dispersions are prepared or after the coatings have dried. Dyes such as Rose Bengal, Eosin Y, Erythrosin and Fluoresceing are convenient for sensitizing coatings such as the coating of Example 1.

The coating of the final product may be any thickness. However, it is preferred that the coating thickness be about 0.0003 to about 0.002 inch thick. For the coating of Example 1, a coating 0.001 inch thick is preferred.

The photoconductor may be suspended in the vehicle in any one of several ways. The simplest way is to dissolve the vehicle in an organic solvent capable of effecting solution and then mixing in the powdered photoconductor. Alternatively, the photoconductor may be dry blended, as by kneading with the vehicle heated to a sufficiently high temperature to render it plastic.

Additional examples of formulations which produce improved compositions for electrostatic printing according to the invention are given in parts by weight as follows:

EXAMPLE 2 Parts Florence Green Seal-8 zinc oxide 125 Pliolite S-S marketed by the Goodyear Tire and Rubber Co., Akron, Ohio 2.0 Piccopale 100 resin 20 Aroclor 1254 plasticizer marketed by Monsanto Chemical Co., Organic Chemicals Division, St.

Louis 7, Missouri 10 Toluene 100 EXAMPLE 3 Florence Green Seal-8 zinc oxide 125 Pliolite S-S 25' Piccopale 100 resin 15 Aroclor 1248 plasticizer 10 Toluene EXAMPLE 4 Florence, Green Seal-8 zinc oxide Pliolite S5 1S Piccopale 100 35 Toluene 160 EXAMPLE 5 Florence Green Seal-8 zinc oxide 12.5 Pliolite S-S 30 Chlorowax-70 (a chlorinated parafl-ln soluble in toluene) marketed by Diamond Alkali Co., 300

Union Commerce Building, Cleveland 14, Ohio 20 Toluene 9 EXAMPLE 6 Parts Florence Green Seal-8 zinc oxide 125 Pliolite S- resin Piccopale 100 resin Lindol (tricresyl phosphate) plasticizer marketed by Celanese Corporation of America, Chemical Division, 180 Madison Avenue, New York 16,

NY. 15 Toluene 118 EXAMPLE 7 Florence Green Seal-8 zinc oxide 60 Pliolite S-5 resin 9 Piccopale 100 resin 6 F21 plasticizer 0.5 Toluene 100 Cellosolve acetate 5 Rose Bengal (CI. 779) dissolved in methyl ethyl ketone 0.24

EXAMPLE 8 Florence Green Seal-8 zinc oxide 13$ Acryloid B-82 resin marketed by Resinous Products Division, Rohm and Haas Company, 222 W. Washington Square, Philadelphia 5, Pennsylvania 35 Ester Gum 8L resin marketed by Hercules Powder Company, Synthetics Dept., Wilmington 99, Delaware 10 Tricresyl phosphate plasticizer 5 Toluene 150 EXAMPLE 9 Florence Green Seal-8 zinc oxide 150 Pliolite S-5 resin Cumar T-3 resin marketed by Barrett Division, Allied Chemical and Dye Corporation, 40 Rector Street, New York 6, NY. 16 Tricresyl phosphate plasticizer 9 Toluene 200 EXAMPLE 10 Florence Green Seal-8 zinc oxide 45 Pliolite S-SD 9 Piccopale 100 6 F-21 plasticizer 0.75 Toluene 175 There have been described improved recording elements for electrostatic printing. The recording elements of the invention are cheap, easily prepared in standard web coating machines, flexible and non-blocking.

What is claimed is:

1. In a composition for coating a substrate to provide thereon a photoconductiveinsulating surface, the combination comprising about 100 to 900 parts by weight of a particulate zinc oxide photoconductor and about 100 parts by weight of an electrically-insulating film-forming vehicle, said vehicle comprising a mixture of about to 62% by weight of a resinous copolymer of styrene with butadiene, said copolymer including at least 80% by weight of said styrene and about 70% to 38% by weight of a hard hydrocarbon produced by the polymerization of unsaturates derived from the deep cracking of petroleum having an average molecular weight of about 1100 and a ball and ring softening point of about 100 C., said paraffin resin comprising a polymerization product obtained by catalytic reacting a mixture of monomers having an average molecular weight of about 90 and consisting essentially of dienes and reactive olefins.

2. A recording element comprising a substrate having thereon a photoconductive insulating coating comprising about 100 to 900 parts by weight of a particulate zinc oxide photoconductor suspended in about 100 parts by weight of an electrically-insulating film-forming vehicle comprising a mixture of about 30% to 62% by weight of a resinous copolymer of styrene with butadiene, said copolymer including at least by weight of said styrene and about 70% to 38% by weight of a hard hydrocarbon produced by the polymerization of unsaturates derived from the deep cracking of petroleum having an average molecular weight of about 1100 and a ball and ring softening point of about 100 C., said parafiin resin comprising a polymerization product obtained by catalytic reacting a mixture of monomers having an average molecular weight of about and consisting essiting essentially of dienes and reactive olefins.

3. The recording element of claim 1 wherein said substrate is paper.

4. A recording element comprising a paper backing having a photoconductive insulating coating thereon comprising about 125 parts by weight of a photoconductive zinc oxide dispersed in an electrically-insulating filmforming vehicle, said vehicle comprising a mixture of about 25 parts by weight of a resinous copolymer of styrene with butadiene, said copolymer including at least 80% by weight of said styrene, 10 parts by weight of a hard hydrocarbon produced by the polymerization of unsaturates derived from the deep cracking of petroleum having an average molecular weight of about 1100 and a ball and ring sofitening point of about C., said paraflin resin comprising a polymerization product obtained by catalytic reacting a mixture of monomers having an average molecular weight of about 90 and consisting essentially of dienes and reactive olefins, and 10 parts by weight of plasticizer.

5. A recording element comprising a paper backing having a photoconductive insulating coating thereon comprising about 45 parts by weight of photoconductive zinc oxide dispersed in an electrically-insulating filmforming vehicle, said vehicle comprising about 9 parts by weight of a resinous copolymer of styrene with butadiene, said copolymer including at least 80% by weight of said styrene, 6 parts by weight of a hard hydrocarbon produced by the polymerization of unsaturates derived from the deep cracking of petroleum and 1.5 parts by weight of a plasticizer.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Raw Materials Index, National Paint, Varnish and Lacquer, Ass. Supplement 9, May 1949, pages 39 and 40.

Wainer: Photographic Engineering, vol. 3, No. 1, pages 12-22 (1952).

Piccopale, American Ink Maker, vol. 32, No. 5, page 31, May 1954.

Whitby: Synthetic Rubber, 557-560, 629-637 and 638-644.

Young et al.: RCA Review, vol. XV, No. 4, pages 469-484.

Modern Plastics, Encyclopedia Issue, September 1955, pages 82 and 84.

Piccopale, Brochure of Pennsylvania Industrial Chemical Corp, no date (12 folded sections).

Wiley (1954) pages UNITED STATES PATENT OFFICE CERTIFICATE OF, CORRECTION Patent No 2,99o 2u9 I June 27, 1961 Julius A, Crumley et al,

It is hereby certified that error appears in the above numbered patentrequirihg correction and that the said-Letters Patent should read as "corrected below.

Column l0 line 40, after "petroleum" insert having an average molecular weight of about 1100 and a ball and ring softening point of about 100 C, a said paraffin. resin comprising a polymerization product obtained by catalytic reacting a mixture of monomers having an averagemolecular weight of about 90 and consisting essentially of dienes and reactive olefins Signed and sealed this 13th day of February 1962c (SEAL) Attest:

ERNEST W. SWIDER- DAVID L. LADD Attesting Officer Commissioner of Patents 

1. IN A COMPOSITION FOR COATING A SUBSTRATE TO PROVIDE THEREON A PHOTOCONDUCTIVE INSULATING SURFACE, THE COMBINATION COMPRISING ABOUT 100 TO 900 PARTS BY WEIGHT OF A PARTICULATE ZINC OXIDE PHOTOCONDUCTOR AND ABOUT 100 PARTS BY WEIGHT OF AN ELECTRICALLY-INSULATING FILM-FORMING VEHICLE, SAID VEHICLE COMPRISING A MIXTURE OF ABOUT 30% TO 62% BY WEIGHT OF A RESINOUS COPOLYMER OF STYRENE WITH BUTADIENE, SAID COPOLYMER INCULDING AT LEAST 80% BY WEIGHT OF SAID STYRENE AND ABOUT 70% TO 38% BY WEIGHT OF A HARD HYDROCARBON PRODUCED BY THE POLYMERIZATION OF UNSATURATES DERIVED FROM THE DEEP CRACKING OF PETROLEUM HAVING AN AVERAGE MOLECULAR WEIGHT OF ABOUT 1100 AND A BALL AND RING SOFTENING POINT OF ABOUT 100*C., SAID PARAFFIN RESIN COMPRISING A POLYMERIZATION PRODUCT OBTAINED BY CATALYTIC REACTING A MIXTURE OF MONOMERS HAVING AN AVERAGE MOLECULAR WEIGHT OF ABOUT 90 AND CONSISTING ESSENTIALLY OF DIENES AND REACTIVE OLEFINS. 